In the development of high-capacity optical communications systems the use of highly mode-selective monochromatic light sources is desirable, and semiconductor lasers are being considered as particularly suitable in this respect. In particular, and with a view towards potentially superior mode selectivity, there is strong interest in semiconductor lasers in which feedback is produced at least in part by a grating arrangement.
Lasers in which feedback is produced by a grating have become known as dynamic single-mode (DSM) lasers, and these, in turn, are classified as distributed-Bragg-reflector (DBR) or distributed-feedback (DFB) lasers depending on placement of the grating: in the case of a DBR laser the grating forms a part of a waveguide adjoining the active region in a direction of light propagation; in the case of a DFB laser the grating is alongside the active region. In the following, attention is directed primarily to DFB lasers.
One possible structure for a distributed-feedback laser comprises a grating which is formed on a substrate prior to the deposition of waveguiding and active layers; see, e.g.,
K. Utaka et al., "Room-temperature CW Operation of Distributed-feedback Buried-heterostructure InGaAsP/InP Lasers Emitting at 1.57 micrometer", Electronics Letters, Vol. 17 (1981), pp. 961-963;
Y. Uematsu et al. "Room-temperature CW Operation of 1.3 micrometer Distributed-feedback GaInAsP/InP Lasers", Electronics Letters, Vol. 18 (1982), pp. 857-858;
T. Matsuoka et al., "CW Operation of DFB-BH GaInAsP/InP Lasers in 1.5 micrometer Wavelength Region", Electronics Letters, Vol. 18 (1982), pp. 27-28; and
L. D. Westbrook et al., "High-quality InP Surface Corrugations for 1.55 micrometer InGaAsP DFB Lasers Fabricated Using Electrion-beam Lithography", Electronics Letters, Vol. 18 (1982), pp. 863-865.
An alternate approach to the formation of a grating consists in forming a grating after deposition of the active layer, followed by cladding layer deposition. This approach has become known as "overgrowth" and is exemplified by the following:
A. W. Nelson et al., "Deformation-free Overgrowth of InGaAsP DFB Corrugations", Electronics Letters, Vol. 19 (1983), pp. 34-36;
M. Kitamura et al., "High-power Single-longitudinal-mode Operation of 1.3 micrometer DFB-DC-PBH LD", Electronics Letters, Vol. 19 (1983), pp. 840-841; and
M. Kitamura et al., "Low-threshold and High-temperature Single-longitudinal-mode Operation of 1.55 micrometer-band DFB-DC-PBH LDs", Electronics Letters, Vol. 20 (1984), pp. 595-596.
In a variant approach, overgrowth is partially etched away to produce a so-called ridge structure which serves for lateral mode confinement. The following items are representative with respect to this approach:
J. E. Bowers et al., "1.55 micrometer Multisection Ridge Lasers", Electronics Letters, Vol. 19 (1983), pp. 523-525;
L. D. Westbrook et al., "Continuous-wave Operation of 1.5-micrometer Distributed-feedback Ridge-waveguide Laser", Electronics Letters, Vol. 20 (1984), pp. 225-226;
H. Temkin et al., "1.55-micrometer InAsAsP Ridge Waveguide Distributed Feedback Laser", Applied Physics Letters, Vol. 45 (1984), pp. 1178-1180;
W. T. Tsang et al., "Heteroepitaxial Ridge-overgrown Distributed Feedback Laser at 1.5 micrometer", Applied Physics Letters, Vol. 45 (1984), pp. 1272-1274; and
H. Temkin et al., "Ridge Waveguide Distributed Feedback Lasers with Electron Beam Defined Gratings", Applied Physics Letters, Vol. 46 (1985), pp. 105-107.
While the above-cited items disclose feedback gratings which are placed in close proximity to the active region, it has also been possible, by means of x-ray lithography and ion-beam-assisted etching, to form a grating in a cladding layer as disclosed by
Z. L. Liau et al., "A Novel GaInAsP/InP Distributed Feedback Laser", Applied Physics Letters, Vol. 46 (1985), pp. 221-223.
One structural feature which is shared by the above-cited items lies in the uniform flatness of layers which make up the disclosed semiconductor lasers. This is contrasted with considerable departure from flatness in the case of semiconductor lasers variously designated as V-groove, buried-crescent, or channeled-substrate lasers and as disclosed, e.g., in the following:
H. Ishikawa et al., "V-grooved Substrate Buried Heterostructure InGaAsP/InP Laser", Electronics Letters, Vol. 17 (1981), pp. 465-467;
R. A. Logan et al., "InGaAsP/InP (1.3 micrometer) Buried-crescent Lasers with Separate Optical Confinement", Electronics Letters, Vol. 18 (1982), pp. 895-896;
J. P. Van Der Ziel et al., "Quaternary 1.5 micrometer (InGaAsP/InP) Buried Crescent Lasers with Separate Optical Confinement", Electronics Letters, Vol. 19 (1983), pp. 113-115; and
D. P. Wilt et al., "Channeled-substrate Buried Heterostructure InGaAsP/InP Lasers with Vapor Phase Epitaxial Base Structure and Liquid Phase Epitaxial Regrowth", Journal of Applied Physics, Vol. 56 (1984), pp. 710-712.
Laser qualities such as, e.g., low threshold and high output power, and established high-yield manufacturing procedures are among factors favoring the use of V-groove buried-heterostructure lasers in optical communications systems. However, such lasers so far have been made only with a Fabry-Perot cavity and, indeed, V-groove structure has been considered as incompatible with the requirements of distributed feedback. Specifically, due to steeply sloping sidewalls, the placement of a grating in the substrate beneath waveguiding and active layers would appear to be difficult indeed. Also, while layer deposition results in a certain amount of filling of the V-groove, layer surfaces remain far from level; moreover, the thickness of deposited layers is nonuniform, with considerably greater thickness along the center line than towards the walls of the V-groove. In view of lack of planarity and nonuniformity of thickness, making a grating after deposition of the active layer would seem to require precise control over the depth of grating grooves--a requirement which remains unmet by practicable methods of manufacture.